At present, there is developed, as a flexible device such as electronic paper, an electronic device using a plastic substrate such as a polycarbonate and a polyethylene terephthalate. From the problem that these plastic substrates are slightly elongated/contracted during heat application, an enhancement of heat resistance (low thermal expansion characteristics) thereof has become an urgent necessity.
Thus, in order to relieve a thermal stress itself applied on a plastic substrate, a production process of an electronic device that is performed at lower temperatures is also developed. One of processes for which a high temperature is mostly required in the production of electronic devices is a process of forming/curing a gate insulating film of an organic transistor, and it is required that the production process of a gate insulating film is performed at lower temperatures.
Here, as the method for forming a gate insulating film at lower temperatures, there are disclosed a method of anodizing the surface of an electrode (see Patent Document 1), a method by a chemical vapor deposition method (see Patent Document 2), and the like. However, a production process of these methods is cumbersome.
Therefore, a material capable of being easily formed into a film by application by a spin coating method, a printing method, and other applying methods, is desired. As an example of production of a gate insulating film by application, there is a method including applying a solution containing poly-4-vinylphenol and poly(melamine-formaldehyde) by spin-coating and then curing the solution at 200° C. (see Non-patent Document 1). However, in this method, the treating temperature is as high as 200° C., and at this temperature, the effects of a thermal expansion of a plastic substrate remarkably materialize, so that it is difficult to apply this method to the production of electronic paper or the like for which a fine pixel is required.
On the other hand, a soluble polyimide is known as one of insulating materials which can be produced at a relatively low temperature and for which high insulation properties can be expected. A polyimide has generally a high thermal decomposition temperature and high electric resistance, so that the polyimide is widely used as an insulating material for an electronic device. For example, there is disclosed a method using a polyimide cured at 180° C. as a gate insulating film for a high-precision organic transistor (see Non-patent Document 2).
In recent years, particularly in a thin-film transistor excellent in mechanical flexibility represented by an organic transistor, for the purpose of reducing the production cost, the formation of an electrode or a wiring by irradiation with an ultraviolet ray having high energy is performed.